The present disclosure relates to an air-fuel ratio controller of an internal combustion engine.
When a fuel injection valve is operated based on a base injection amount, which is an operation amount of an open-loop control that sets the air-fuel ratio to a target value, the actual air-fuel ratio is deviated from the target value due to deviations in the injection performance of the fuel injection valve from the reference performance or deviations of the actual cylinder intake air amount from a cylinder intake air amount used to calculate the base injection amount. In this regard, when the fuel injection valve is operated through a feedback control in addition to the open-loop control, which uses the base injection amount, the feedback operation amount compensates for the deviation of the actual air-fuel ratio from the target value described above (error of air-fuel control caused by base injection amount). Additionally, a known air-fuel ratio control learns a compensation amount that compensates for an error of the air-fuel ratio control caused by the base injection amount as a learning value.
Japanese Laid-Open Patent Publication No. H8-4579 describes an example of an air-fuel ratio controller that learns a learning value. The device updates the learning value based on a feedback operation amount of the air-fuel ratio when the base injection amount is corrected by an increase of a warm-up increase amount (low temperature increase amount), which is the amount of fuel increased when the temperature is low. The device updates the learning value based on a value obtained by adding a correction amount corresponding to the increase correction to a correction rate of the base injection amount corresponding to the feedback operation amount.
As described above, when the increase correction is performed on the base injection amount, a portion of the increased fuel does not collect on, for example, a cylinder wall surface and contributes to enrichment of the exhaust air-fuel ratio. The feedback operation amount is changed in correspondence with the portion of the fuel that is increased by the low temperature increase amount and contributes to the enrichment of the exhaust air-fuel ratio.
The proportion of the fuel contributing to the enrichment of the exhaust air-fuel ratio changes in accordance with the temperature of the cylinder wall surface of the cylinder even when the increase correction rate of the base injection amount corresponding to the low temperature increase amount is the same. Thus, as performed in the above device, only the addition of the correction amount corresponding to the low temperature increase amount to the correction rate of the base injection amount corresponding to the feedback operation may cause the learning value to be mislearned, that is, the updating of the learning value is largely affected by the portion of the fuel increased by the low temperature increase amount and contributing to the enrichment of the exhaust air-fuel ratio, depending on the temperature of the cylinder wall surface. More specifically, the learning value may be updated to a value that is largely deviated from the value appropriate for compensating for an error of the air-fuel control caused by deviations in the injection performance of the fuel injection valve from the reference performance or deviations of the actual cylinder intake air amount from the cylinder intake air amount used to calculate the base injection amount.